The present invention relates to a process for preparing urethanes and carbonates and, more particularly, relates to a new and useful process for preparing urethanes from amines, carbon dioxide and a hydrocarbyl halide and for preparing carbonates from alcohols, carbon dioxide and a hydrocarbyl halide. The present invention also relates to polymers prepared from such urethanes and/or carbonates.
2. Prior Art
Urethanes and carbonates are typically synthesized by the reaction of a primary amine or an alcohol with phosgene to form an isocyanate or carbonate salt. Thereafter, the isocyanate or carbonate is reacted with an alcohol to form the corresponding urethane or carbonate. Phosgene is very toxic and thus requires very careful handling from a product and worker safety standpoint. Isocyanates are sensitizers and are extremely toxic as well. Preparing urethane and carbonate products without using phosgene and in an economical manner, and preparing urethane products without generating isocyanates would be an achievement of considerable significance in the art.
U.S. Pat. No. 4,467,089 discloses the preparation of certain carbamic acid derivatives (carbonates and carbamate esters) by the simultaneous reaction of a secondary amine and a tertiary amine with carbon dioxide to produce corresponding tertiary amine salts of N-substituted carbamic acid. The secondary and tertiary amines are brought together in equimolar proportions in the presence of excess carbon dioxide under mild conditions. The secondary amine reacts with CO.sub.2 in the presence of the tertiary amine to form the corresponding disubstituted tertiary ammonium carbamate salt. The salt is described as being useful as heat activatable delayed action catalysts, especially for use in polyurethane formulations.
Yoshida et al, Bull. Chem. Soc. Jpn., 62, 1534-38 (1989) discloses preparation of urethanes from amines, carbon dioxide and alkyl halides. However, under the reaction conditions specified therein, yields of urethane product are poor as nitrogen derived products are the predominant product.
In Chemistry Express, Vol. 1, No. 4, pp 224-227 (1986), Kinki Chemical Society, Japan, it is disclosed that primary and secondary amines absorb CO.sub.2 to form carbamic acid amine salts and that when an equivalent of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) is added, additional CO.sub.2 is absorbed to form the DBU-carbamate salt. The DBU-carbamate salt when reacted in a nonpolar aprotic solvent with an alkylating agent forms a carbamate ester (urethane). Yield and selectivity of the urethane product are highly dependent on the nature of the alkylating agent. When dibutylamine is reacted with CO.sub.2 in the presence of DBU and the resulting DBU-carbamate salt is reacted with butyl chloride as the alkylating agent, a yield of only 17% is realized. With butyl bromide, the yield is 86%. However, when the reaction with butyl bromide was repeated, it was observed that this yield could be achieved only if the reaction was allowed to continue for an extensive period of time, such as from about 18 to about 30 hours. Thus, this reaction, like the reaction disclosed by Yoshida et al, is not commercially practicable.
It has now been discovered that unexpectedly high yields can be achieved in a commercially practicable period of time, i.e. from one-fourth to one-half of the time set forth above, by conducting the reaction in a polar aprotic solvent and in the presence of a strongly basic nitrogen-containing base selected from amidine- and guanidine-type bases.